Electromagnetic transducers are known in general for example from WO 95/14363 or in particular with linearization of the characteristic curve by inserting a permanent magnet, for example from EP O 774 880 or from U.S. Pat. No. 4,680,492. Such transducers are primarily used as signal generators or door buzzers. It is a characteristic of these applications that the nonlinearity of the power line current curve either causes no disturbance (e.g. due to heavy damping of the harmonics) or that the nonlinearity becomes tolerable due to premagnetization and minor control.
Diaphragm loudspeakers in a planar configuration are known as piston radiators, for example from U.S. Pat. No. 5,539,835 or U.S. Pat. No. 4,928,312, or in the multiresonance configuration as bending wave radiators for example from WO 97/09842 or DE 197 57 097, and in addition to the sturdy, rigid plate (diaphragm) with a holder they comprise a drive system (e.g. one or several drivers) which provide excitation power to the plate at one or several points.
Beyond that WO 97/17818 or U.S. Pat. No. 5,638,456 propose for example piezoelectric drivers which, although they are very sturdy, in practice are always too weak for large plates.
Even though electrodynamic drives develop sufficient power and deflection, they have however a setting problem in connection with the plate coupling. The usual sandwich plates made of different types of bonded materials are very light and unbending, but do not keep their shape over time. Particularly the layers of adhesive used to produce the sandwich plates change their consistency. Constant gravity for example produces a certain creep and flow direction. Beyond that thermal stresses during operation lead to local softening with irreversible shape changes. This in turn causes the coil which is attached to the plate to shift from its original position.
Each relative misalignment between the coil directly attached to the plate and the magnet system that is attached farther away creates displacement components which tilt the coil's axis from its normal position or shift the coil into an eccentric position. This can cause the voice coil to touch the walls of the annular gap in the magnet system and thereby render the drive unusable.
The operation of bending wave radiators has the further problem in which the usual drivers perform an undesirable pumping movement, because bending without “pumping” is desirable in bending wave radiators, as opposed to piston radiators.
Furthermore the drivers named so far do not permit any edge excitation during bending wave operation. But this excitation position is necessary when using transparent plates, or plates on which both sides are used as an image field. Even though the electrodynamic drivers known for example from U.S. Pat. No. 4,392,027 or DE 198 21 860, which exert power normal to the plate surface, can be cost-effectively produced, they have the disadvantage of a relatively large construction depth and need a relatively large surface for support by an external bead. Furthermore it is precisely the edge area of the plate which creates a problem for the long-term stable adjustment of the voice coil position with respect to the external bead.